An Industrial Perspective on Challenges in the Lifetime Prediction of Organic Coatings: A Century Retrospective Review

Abstract

There has been a dedicated effort by multiple groups to determine which performance attributes of coatings, such as adhesion, electrochemical properties, and appearance will give an indication of the overall performance of a coating. Complicating lifetime performance predictions of coatings are the many variables that consistently change in differing industrial markets—substrates, pretreatments, coating chemistries, and coating layers to name a few, with coatings failing in different manners. While the focus of this paper is the lifetime prediction of corrosion performance in atmospheric corrosion scenarios, it should be noted that there are other failure modes (Table 1), beyond corrosion, which may be considered “end of life” by users. These include appearance changes such as ultraviolet (UV) degradation resulting in color, gloss loss, coating erosion and loss, or changes in adhesion strength. While these failure modes are out of scope for this paper, it must be realized that the chemical changes occurring during UV exposure may significantly impact the coating performance in the area of corrosion. To be of value, lifetime prediction must accurately determine the coating lifetime when under real-world conditions, noting that accelerated testing protocols used in lifetime prediction studies need to be interpreted with caution, as many do not accurately replicate the modes of failure that occur during real-world use. Complicating this is the fact that coatings are studied in the laboratory under ideal conditions, with well-prepared substrates, and properly applied and cured coatings at controlled dry coating thicknesses. Coating lifetimes in the field are often not determined by this “Best Case Scenario.” Instead, coatings tend to fail in use due to substrate preparation issues, application defects, incomplete cure, damage during use, along with a variety of other unknown unknowns.